The present invention relates to a semisubmersible multipurpose unit (MPU) adapted for the drilling and completing of platform-based offshore oil and gas wells and the servicing of offshore oil and natural gas production platforms, subsea wells, and other subsea infrastructure using a multipurpose tower in water depths up to 10,000 feet.
More specifically, the present invention relates to a semisubmersible MPU which can be secured to different types of production platforms, such as a tension leg platform (TLP), a deep draft caisson vessel (SPAR), a fixed platform, a compliant tower, a semisubmersible production vessel or a floating vessel, and which utilizes a unique multipurpose tower. Using the semisubmersible MPU""s construction crane, the multipurpose tower can be constructed and removed from the semisubmersible MPU and erected on a production platform as required, in order to enable a tender drilling operation to be conducted. Additionally, when the multipurpose tower is erected on the semisubmersible MPU, the unique semisubmersible MPU with multipurpose tower can be used for drilling operations, well completions, maintenance and work-over operations on subsea wells, as well as the installation, maintenance and removal of other subsea infrastructures, such as manifolds, gathering lines, risers and templates.
It is very expensive to provide a production platform with a powerful drilling rig and adequate space for all the drilling equipment needed to drill a well safely and store drilling equipment and materials in an environmentally conscientious manner, particularly where the equipment includes drilling risers, casings, solid waste management equipment, well stimulation, completion assemblies, alloy tubulars, and drilling and completion fluids. Mono-hull and semisubmersible tenders have often been called into service to provide the required space needed on a rig and/or platform during the initial drilling phase of an oil and gas lease. Problems have traditionally existed in that most tenders cannot be kept alongside a platform in a constant spaced relationship during inclement weather without colliding with the platform or risking the safety of the offshore workers. In addition, most tenders can only be used for drilling and completing wells from the production or wellhead platform using a drilling equipment set. Expensive mobile offshore drilling units such as semisubmersibles, jackups and drill ships often have handled exploration, development and well intervention operations in many different weather, water depth, and regulatory scenarios for subsea or mud line suspension wells. This diversity of operating environments has required operators to use many different types of drilling, completion, and work-over rigs. Operators, therefore are required to utilize many short-term contracts, rather than fewer long-term contracts with more versatile rig designs. The requirement to perform work through short-term contracts has impeded performance from operational, safety, and environmental perspectives. It also has impeded the construction of newer, more efficient and environmentally sensible rig designs since drilling contractors have not been able to earn attractive returns on investment due to poor utilization rates and due to the fact that oil and gas companies cannot justify long-term work programs for a rig with a narrow scope of work versatility. Additionally, tenders have not been able to remain in a connected operational capacity during inclement weather without risking the lives of the offshore employees and the damage and potential loss of equipment. The operational windows have been significantly reduced with bad weather and strong loop current conditions, particularly when the environmental load is up to and of a 1-year winter storm or tropical storm event. See U.S. Pat. Nos. 4,065,934, and 4,156,577, which are hereby incorporated by reference, and provide basic information on current tender design. Most tenders must be towed away to a safe location in the case of a tropical storm or extreme weather. This towing adds considerable expense to the drilling contractor and to the customer.
It generally has been believed to be impossible to moor safely a tender alongside a floating production platform in water depths exceeding one thousand feet during harsh weather conditions, such as 10-year storms, and remain operational for long periods of time, such as up to one year, or anytime during hurricane season.
A need has long existed for a semisubmersible MPU that supports platform-based and subsea wells, as well as the related infrastructure. The present invention is designed to provide great versatility with respect to various types of jobs, including for example:
1. shallow and deepwater semisubmersible tender for platform well work;
2. shallow and deepwater semisubmersible tender for platform infrastructure work;
3. shallow and deepwater well work in subsea wells;
4. shallow and deepwater construction support for subsea developments; and
5. shallow and deepwater support for early production operations.
Historically, all of these jobs have been performed by up to seven different types of rigs. This situation has required operators to contract several different types of rigs on short-term contracts. Short-term contracts are those contracts of less than two years, normally less than six months. These short-term work scopes have resulted in highly cyclic rig rates, lower profit margins, and a highly transient offshore work force. The effect has been inconsistent performances and an increased risk of operational problems. The end result is borne by the end user in the form of higher energy costs.
The present invention has been created to provide a semisubmersible MPU with up to 30,000 square feet of additional space, over 8000 barrels of liquid storage capacity, and a self-erecting multipurpose designed tower (MPT) that can be assembled offshore and temporarily secured to either the production platform or the semisubmersible MPU. Further, it has the ability to maintain a constant distance from a production platform while synchronizing to its low and average movement frequencies. This enables the semisubmersible MPU to imitate and act in parallel to the mooring watch pattern of the platform to which it is tied, which has either a figure eight mooring watch pattern or an elliptical mooring watch pattern. The MPU is able to be sustained without damage while moored in an environmental load of wind, current, and wave forces of a 100-year cyclonic storm (such as a hurricane) in the 100-year extreme weather standby position and can also be sustained without damage in a 10-year storm in standby tendering position.
The present invention is related to a semisubmersible tender with conventional derrick equipment set. This MPU with MPT has significant environmental and safety advantages over known semisubmersible tenders and known methods for handling drilling operations and is designed for zero discharge, including the processing and clarification of rainwater and solid wastes such as drill cuttings.
The present invention includes the semisubmersible MPU with multipurpose tower and the semisubmersible MPU with tower mooring system utilizing pre-set anchors, as well as various methods for servicing wells and other subsea operations including, but not limited to, semisubmersible tendering to a deep-water production platform for assisting in the drilling and recovery of oil and gas, in weather that can be up to a 10-year storm and maintaining a standby position in weather up to a 100-year hurricane. The tower can be erected or dismantled using the semisubmersible tender""s construction crane, which allows the multipurpose tower to be shared between semisubmersible MPUs and various production platforms, further increasing the versatility and economic advantages.
The present invention is directed to solving one or more of the above problems by providing a semisubmersible MPU and unique multipurpose tower combination for facilitating installation, operational support, drilling, completing and maintaining wells, and/or removal of drilling and completion equipment from a production platform while compensating for platform motions in at least one plane. The present invention also is directed to solving problems associated with drilling and completing wells and performing well maintenance operations on subsea wells located in proximity of or remotely from a production platform, facilitating the installation, operational maintenance, and/or removal of subsea infrastructure such as templates, manifolds or single risers.
The present invention relates to a semisubmersible MPU with a multipurpose tower (MPT), a crane and a mooring system. The semisubmersible MPU with multipurpose tower has a lightship displacement of less than 20,000 short tons. More specifically, the semisubmersible MPU comprises a deck, a self-erecting multipurpose tower removably secured to the deck, a drawworks for hoisting a drawworks line, a top drive mounted on the tower, hoisting blocks secured to the tower, a control cabin connected to the tower, and a heave compensator. Active heave compensation also can be incorporated into the design using a dynamically controlled drawworks. The MPT comprises at least two members of the group including a base structure, a tower, and a crown. The crane is removably secured to the deck. The hull shape and general configuration of the semisubmersible MPU is designed to result in a combined environmental load of less than 1000 kips in a 100-year extreme weather condition. The semisubmersible MPU further comprises a plurality of pontoons connecting a plurality of the supports connected to the deck, and at least two hawsers for connecting the semisubmersible tender to the production platform. Each hawser has a length, which is selected from the group: the length of the semisubmersible tender, the semisubmersible tendering distance, the length of the production platform, and combinations thereof. The hawsers have sufficient elasticity to accommodate the wave frequency motions between the production platform and the semisubmersible tender, and sufficient stiffness and tension to synchronize the mean and low frequency movement between the production platform and the semisubmersible MPU under an environmental load produced during a storm having a designation of up to a 10-year storm in the semisubmersible tendering position.
This invention also comprises a mooring system that permits the semisubmersible MPU with multipurpose tower to remain connected to the platform, while the hawsers remain slack during a storm designated as at least a 10-year storm for the semisubmersible MPU in the semisubmersible tender standby position. The semisubmersible MPU further has connecting means for securing a first end of each hawser to the semisubmersible MPU, and a hawser guidance system that can be a conical horn to direct each hawser to the production platform or a series of fairleads or sheaves.
The mooring system for the semisubmersible MPU with multipurpose tower combination is an at least 6-point mooring system for the semisubmersible MPU which uses at least 6 anchors and at least 6 mooring lines, each mooring line consisting of: a first length of steel wire rope or chain secured to each of the anchors, a length of polymer rope secured to the first length of steel wire rope or chain, a second length of steel wire rope having a first and second end, wherein the first end is secured to the length of polymer rope and the second end is secured to the semisubmersible MPU.
Each mooring line has sufficient elasticity, stiffness and strength to accommodate load on the semisubmersible MPU under an environmental load produced by up to and by a 10-year storm in the semisubmersible tendering position, and further wherein the mooring lines have a strength to withstand the environmental load produced by up to a 100-year extreme weather condition when the semisubmersible MPU is moved to a 100-year extreme weather condition standby position.
The semisubmersible MPU further has means for creating global equilibrium between the production platform""s mooring means and an at least 6-point mooring system of the semisubmersible MPU.
The MPT of the semisubmersible MPU with multipurpose tower is a multipurpose design which is preferably assembled on the water, secured to the semisubmersible MPU deck, and then used for well operations such as well drilling, completion, maintenance, and well work-over and other subsea infrastructure operations or, alternatively, the multipurpose tower is erected as part of the drilling equipment set that is placed on the production platform and then used for well operations such as drilling, completion, maintenance and workover of dry tree wells.
Additional objects, advantages and novel features of the invention will be set forth in part of the description which follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by practice of the invention.